Diadenosine tetraphosphate (hereinafter sometimes referred to as AP4A) tetrasodium salt is a substance which is useful as a medicine and a starting material for the production of a medicine. AP4A tetrasodium salt has various physiological functions, such as promoting the synthesis of DNA on BHK cells suffering from G.sub.1 inhibition (F. Grummt, Proc. Natl. Acad. Sci., 75, p. 371, 1987), inhibiting phosphorylation (P. F. Maness et al., J. Biol. Chem., 258, p. 4055, 1983) and inhibiting platelet aggregation (M. J. Harrison et al., FEBS Letters, 54, p. 57, 1975).
With respect to a method for producing AP4A, a method of freeze-drying AP4A as a sodium salt to thereby give AP4A as a solid product is known. Further, there has been known another method for producing AP4A tetrasodium salt by adding a hydrophilic organic solvent to an aqueous solution of AP4A tetrasodium (salt) in an amount of about fourfold of the aqueous solution and recovering the precipitate thus formed, to obtain the AP4A tetrasodium salt in the form of a solid powder.
However, the conventional AP4A tetrasodium salt, which is in the form of either a solid or a powder, is an amorphous anhydride and thus physically unstable. Moreover, it has an extremely high hygroscopicity. Due to these characteristics, it suffers from such problems as having poor storage stability, because it easily undergoes coloration, deliquescence, etc. In particular, if the coloration or deliquescence of solid or powdery AP4A tetrasodium salt occurs, it cannot be used as a medicine or a starting material for the production of a medicine. It is therefore required to strictly control the humidity, etc. of the storage environment. Accordingly, the conventional AP4A tetrasodium salt is disadvantageous for handling as a medicine or a starting material for the production of a medicine.
The present invention, which has been completed under these circumstances, aims at solving the above-mentioned problems accompanying the conventional solid or powdery AP4A tetrasodium salt by providing a novel AP4A tetrasodium salt which can be stored in a physically stable state and under a wide range of humidity conditions.